A shield machine includes a shield machine main body having a body, a plurality of shield jacks configured to cause the shield machine main body to move ahead, and an excavator provided on a front end side of the shield machine main body to excavate natural ground. In a general shield machine, the body of the shield machine main body is formed to have a cylindrical shape, and the excavator includes a plurality of cutter bits provided on a front surface of a cutter head having the same diameter as the body. The cutter head is supported by a front end side portion of the shield machine main body to be rotatable around a central axis of the shield machine main body, and is rotated by a driving machine, such as a plurality of hydraulic motors. The shield machine excavates a tunnel of a circular cross section by causing the shield machine main body to move ahead (drill ahead) while rotating the cutter head.
In recent years, shield machines capable of excavating tunnels of various cross-sectional shapes, such as a rectangular cross section and an oval cross section in addition to the circular cross section, have been developed. Examples of such shield machines are described in Patent Documents 1 to 3.
In the shield machine of Patent Document 1, a disc cutter is supported to be rotatable around a central axis of the shield machine main body, and a planetary cutter is rotatably supported by a disc main body of the disc cutter. In accordance with this shield machine, when the disc main body is rotated, the planetary cutter revolves integrally with the disc main body, and rotates in sync with the rotation of the disc main body by a planetary gear mechanism. Thus, the shield machine can excavate a tunnel of a predetermined cross-sectional shape by the disc main body and the planetary cutter.
In the shield machine of Patent Document 2, four spokes radially extend from a small-diameter main cutter provided to excavate a center of a tunnel and are provided with extensible arms, respectively, and each of the extensible arms is provided with a corner cutter and a hydraulic motor configured to cause the corner cutter to rotate. The extensible arms are configured to be extensible and retractable in a radial direction by hydraulic jacks. A peripheral portion of a unique cross section is excavated by causing the corner cutters to rotate while extending and retracting the extensible arms, and a center portion of the cross section is excavated by the main cutter.
In the shield machine of Patent Document 3, a fixed shaft is provided at a central axis of the shield machine main body, and a first casing is provided to be rotatable around the fixed shaft. A second casing is provided to project forward from a tip end portion of the first casing. In the first casing, a fixed gear provided on the fixed shaft, a planetary gear engaging the fixed gear, and a first gear wheel engaging the planetary gear are rotatably supported. Moreover, in the second casing, a second gear wheel engaging the first gear wheel is fixedly provided, and a third gear wheel engaging the second gear wheel is rotatably supported. An excavating cutter is fixedly provided on the third gear wheel. By a casing drive motor and a cutter drive motor provided in the shield machine main body, the first and second casings are rotated around the fixed shaft. Then, in sync with this rotation, the cutter is rotated around a shaft member, and the cutter is independently rotated to rotate. Thus, a predetermined cross section is excavated.
Patent Document 1: Japanese Patent No. 2898968
Patent Document 2: Japanese Laid-Open Patent Application Publication 2001-55890
Patent Document 3: Japanese Laid-Open Patent Application Publication Hei 9-119288